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Feldspar
: . | formula = KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8 | molweight = | color = pink, white, gray, brown | habit = | system = triclinic or monoclinic | twinning = tartan, carlsbad, etc | cleavage = two or three | fracture = along cleavage planes | tenacity = | mohs = 6 | luster = vitreous | polish = | refractive = | opticalprop = | birefringence = first order | dispersion = | pleochroism = none | fluorescence= | absorption = | streak = | gravity = | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = opaque | other = exsolution lamellae common | references = }} Ferroan Anorthosite #60025 (Plagioclase Feldspar). Collected by Apollo 16 from the Lunar Highlands near Descartes Crater. This sample is currently on display at the National Museum of Natural History in Washington, D.C., USA. (Unknown scale.)]] .]] }} Feldspars (KAlSi3O8 - NaAlSi3O8 - CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust.Feldspar. What is Feldspar? Industrial Minerals Association. Retrieved on July 18, 2007. Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock."Metamorphic Rocks." Metamorphic Rocks Information. Retrieved on July 18, 2007 Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite.Blatt, Harvey and Robert J. Tracy, Petrology, Freeman, 2nd ed., 1996, pp. 206-210 ISBN 0-7167-2438-3 Feldspars are also found in many types of sedimentary rock."Weathering and Sedimentary Rocks." Geology. Retrieved on July 18, 2007. Etymology Feldspar is derived from the German Feld, "field", and Spath, "a rock that does not contain ore". "Feldspathic" refers to materials that contain feldspar. The alternate spelling, felspar, has now largely fallen out of use. Compositions This group of minerals consists of framework tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers: Potassium-Feldspar (K-spar) endmember KAlSi3O8 Albite endmember NaAlSi3O8 Anorthite endmember CaAl2Si2O8 Solid solutions between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called plagioclase, or more properly plagioclase feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the earth. Albite is considered both a plagioclase and alkali feldspar. In addition to albite, barium feldspars are also considered both alkali and plagioclase feldspars. Barium feldspars form as the result of the replacement of potassium feldspar. Alkali feldspars The alkali feldspars are as follows: * orthoclase (monoclinic),"The Mineral Orthoclase." Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008. — KAlSi3O8 * sanidine (monoclinic)"Sanidine Feldspar." Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008. —(K,Na)AlSi3O8 *microcline (triclinic)"Microcline Feldspar." Feldspar Amethyst Galleries, Inc. Retrieved on February 8, 2008. — KAlSi3O8 *anorthoclase (triclinic) — (Na,K)AlSi3O8 Sanidine is stable at the highest temperatures, and microcline at the lowest. Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye.Ralph, Jolyon & Ida. "Perthite." Perthite Profile on mindat.org Retrieved on February 8, 2008. Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope. Plagioclase feldspars The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses): * albite (0 to 10) — NaAlSi3O8 * oligoclase (10 to 30) — (Na,Ca)(Al,Si)AlSi2O8 * andesine (30 to 50) — NaAlSi3O8 — CaAl2Si2O8 * labradorite (50 to 70) — (Ca,Na)Al(Al,Si)Si2O8 * bytownite (70 to 90) — (NaSi,CaAl)AlSi2O8 * anorthite (90 to 100) — CaAl2Si2O8 Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase solid solution are complex compared to the gap in the alkali feldspars. The play of colours visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae. Barium feldspars The barium feldspars are monoclinic and comprise the following: * celsian — BaAl2Si2O8 * hyalophane — (K,Na,Ba)(Al,Si)4O8 Feldspars can form clay minerals through chemical weathering. Uses * Feldspar is a common raw material in the production of ceramics and geopolymers. * Feldspars are used for thermoluminescence dating and optical dating in earth sciences and archaeology * Feldspar is one of several abrasive ingredients in Bon Ami, a brand of household cleaner in the USA. * Potassic feldspars are used frequently in K-Ar dating and argon-argon dating, with applications to both geochronology and thermochronology. In 2005, Italy was the top producer of feldspar with almost one fifth of world share, followed by Turkey, China and Thailand—reports the International Monetary Fund. See also *List of minerals *List of countries by feldspar production References *Bonewitz, Ronald Louis. (2005). Rock and Gem, New York: DK Publishing. ISBN 978-0-7566-3342-4 ar:فلدسبار bs:Feldspat ca:Feldspat cs:Živce cy:Ffelsbar da:Feldspat de:Feldspat et:Päevakivi el:Άστριοι es:Feldespato eo:Feldspato eu:Feldespato fa:فلدسپات fr:Feldspath gl:Feldespato ko:장석 hr:Feldspat is:Feldspat it:Feldspato he:פצלת השדה lv:Laukšpats lt:Lauko špatas hu:Földpátok nl:Veldspaat ja:長石 no:Feltspat nn:Feltspat pl:Skaleń pt:Feldspato ro:Feldspat ru:Полевые шпаты simple:Feldspar sk:Skupina živca sl:Glinenec sr:Фелдспат sh:Feldspat fi:Maasälpä sv:Fältspat th:เฟลด์สปาร์ tr:Feldspat uk:Польовий шпат vi:Fenspat zh:长石 Category:Feldspar Category:Tectosilicates Category:German loanwords Category:Triclinic minerals Category:Monoclinic minerals